A known on-board evaporative emission control system for a motor vehicle comprises a vapor collection canister that collects volatile fuel vapors generated in the headspace of a fuel tank by the volatilization of liquid fuel in the tank and a purge valve for periodically purging fuel vapors to an intake manifold of the engine. A known type of purge valve, sometimes called a canister purge solenoid (or CPS) valve, is under the control of a microprocessor-based engine management system, sometimes referred to by various names, such as an engine management computer or an engine electronic control unit.
During conditions conducive to purging, the purge valve is opened by a signal from the engine management computer in an amount that allows intake manifold vacuum to draw fuel vapors that are present in the tank headspace and/or stored in the canister for entrainment with combustible mixture passing into the engine's combustion chamber space at a rate consistent with engine operation so as to provide both acceptable vehicle driveability and an acceptable level of exhaust emissions.
Certain governmental regulations require that certain motor vehicles powered by internal combustion engines which operate on volatile fuels such as gasoline, have evaporative emission control systems equipped with an on-board diagnostic capability for determining if a leak is present in the evaporative emission space.
One known type of vapor leak detection system for determining integrity of an evaporative emission space performs a leak detection test by positively pressurizing the evaporative emission space using a positive displacement diaphragm pump. The diaphragm is reciprocated to create test pressure. Commonly owned U.S. Pat. No. 6,192,743, issued Feb. 27, 2001, discloses a module comprising such a pump.
Known test methods include creating superatmospheric pressure in the closed space being tested and detecting changes that are indicative of leakage. One method comprises measuring a characteristic of pump operation. An example of a time-based measurement is a measurement of how frequently a diaphragm pump must be cycled in order to maintain pressure. Other methods of measurement are pressure-based, such as measuring the rate at which pressure decays. Those methods can provide accuracy when ambient conditions are relatively stable, such as when a vehicle has been parked for an extended period of time. Less stable conditions may impair accuracy of measurements. The dynamics of operating a vehicle may prevent a leak test method from providing consistently accurate results. For example, movement of liquid fuel in a tank, i.e. fuel slosh, might create certain pressure anomalies that could give a false result for a leak test.
The inclusion of various filters, both electrical and mechanical, may mitigate the effects of such anomalies. Even with the presence of such aids, it is believed that further improvement toward assuring consistent accuracy of test results is desirable, and it is toward that objective that the present invention is directed.
Commonly owned pending U.S. patent application Ser. No. 09/896,247, filed 29 Jun., 2001, discloses a system and method that compensates for changes in the output efficiency of a pump due to factors such as temperature, age, friction, etc., so that a leak test can be performed and completed within a specified window of time as the pump efficiency changes. The pump is operated in a manner that creates a succession of pressurizing pulse bursts. Each burst contains a number of pressurizing pulses corresponding to the number of times that the pump is stroked, and the bursts are separated by time intervals during which the pump is not stroked. The invention of that patent application concerns self-compensation for changing pump efficiency as the pump ages.